Purpose: Stereotactic body radiation therapy (SBRT) is commonly delivered utilizing 6-degrees of freedom (6DOF) couches. National guidelines continue to expand the indications for SBRT, but not all centers have 6DOF capability. The aim of this study was to quantify 6DOF corrections clinically utilized across common SBRT disease sites, and evaluate dosimetric consequences if 6DOF was not available.
Methods: Sixty-six patients were randomly selected who were treated with SBRT using a 6DOF couch. Treatments were stratified by disease site (brain, lung, prostate, spine, rib, liver/pancreas, and adrenal). Cone-beam computed tomography (CBCTs) were collected for all treatments. The average rotational correction per disease site was calculated. For each site two patients were randomly selected for assessment of dosimetric impact when 6DOF rotational corrections were not applied. The average rotational corrections were applied to the CT and contours using MIM (v.7.0.6) and recalculated using the dynamic planning feature in Pinnacle (v16.2).
Results: In (pitch/roll/yaw) coordinates, average degrees of rotational correction used for brain (1.17/0.93/1.07), lung (1.44/1.34/0.99), prostate (1.42/0.64/0.63), spine (0.92/0.90/0.79), rib (1.48/1.09/1.23), liver/pancreas (1.36/1.31/1.28), and adrenal (1.13/1.51/1.14) were calculated. SBRT tobrain and liver/pancreas patients showed the most change in PTV coverage (2.0-4.0% loss) and were dependent on location and tumor size, while spine and adrenal showed increases in organs at risk (OAR) dose for the spinal cord (122 cGy) and bowel (170 cGy). For lung, prostate, and rib there were minimal-moderate dosimetric impact on PTVs and OARs (0.5-1%, 0.9-2%, and 1.3-2.3% loss for PTV, respectively).
Conclusion: There were variable tradeoffs for the delivery of SBRT without 6DOF technology of minimal to moderate clinical significance. Disease sites that may be safest to deliver SBRT without 6DOF are likely prostate, peripheral lung, and non-spine bone metastases. Future work to predict expected benefit of 6DOF or adaptive treatments at planning are underway.
Radiation Therapy, Stereotactic Radiosurgery, Patient Positioning